Concomitant Light-Reversible Magnetic Response in Multiferroic Oxide Heterostructures for Multiphysics Applications

The concept of multiphysics, where materials respond to diverse external stimuli, such as magnetic fields, electric fields, light irradiation, stress, heat, and chemical reactions, plays a fundamental role in the development of innovative devices. Nanomanufacturing, especially in low-dimensional systems, enhances the synergistic interactions taking place on the nanoscale. Light–matter interaction, rather than electric fields, holds great promise for achieving low-power, wireless control over magnetism, solving two major technological problems: the feasibility of electrical contacts at smaller scales and the undesired heating of the devices. Here, we shed light on the remarkable reversible modulation of magnetism using visible light in epitaxial Fe3O4/BaTiO3 heterostructure. This achievement is underpinned by the convergence of two distinct mechanisms. First, the magnetoelastic effect, triggered by ferroelectric domain switching, induces a proportional change in coercivity and remanence upon laser illumination. Second, light–matter interaction induces charged ferroelectric domain walls’ electrostatic decompensations, acting intimately on the magnetization of the epitaxial Fe3O4 film by magnetoelectric coupling. Crucially, our experimental results vividly illustrate the capability to manipulate magnetic properties using visible light. This concomitant mechanism provides a promising avenue for low-intensity visible-light manipulation of magnetism, offering potential applications in multiferroic devices.

S1 High-Resolution X-ray Diffraction Configurations for Light-Induced Domain Switching Determination.
Different diffraction conditions are required to study the ferroelectric domain switching of the BaTiO3 crystal, always following Bragg's law.For this purpose, a six-circle diffractometer is used, whose geometry and motor motions are capable of monitoring the light reversibility of pure in-plane and out-of-plane ferroelectric domains.The instrumental design and thanks to the beam quality with a huge signal-to-noise ratio provided by The European Synchrotron (ESRF), valuable data is provided on the in-situ behavior of the BaTiO3 single crystal and on the magnetostructural coupling with the Fe3O4 layer operated by light.As illustrated in Figure S2, we have conducted in-plane hysteresis loops while varying the laser power over a range from 5 mW to 50 mW.These measurements were carried out with the magnetic field aligned at different sample rotation angles, specifically at Θ=0° (Figure S2a), Θ=45° (Figure S2b), and Θ=90° (Figure S2c), under both dark and illuminated conditions.Significant findings from these foundational experiments confirm our ability to systematically regulate the magnetic behavior induced by light through controlled variations in light power.The observed relationship demonstrates a linear correlation between the magnetic behavior of the Fe3O4/BaTiO3 heterostructure -that is, HC, squareness (Mr/Ms), and saturation magnetization (Ms)-, and the applied light power across the examined range (see  BaTiO3 undergoes distinct structural transitions with varying temperatures.Below 200 K, it exhibits a rhombohedral structure, followed by an orthorhombic structure between 200 and 275 K. From 275 to 400 K, it displays a tetragonal structure, and above this value, it is cubic 1 .Notably, only the cubic structure displays paraelectric characteristics.The structural parameters used for the simulation of the crystalline structures that can occur as a function of temperature are detailed in Figure S3.

S4 Investigating Light Power Dependency for Robust Light-Induced Magnetic Control in Multiferroic Systems
It is essential to examine the variations in the crystalline domain and lattice parameters of the Fe3O4/BaTiO3 heterostructure with light as they trigger relevant conclusions to properly predict the origin of the observed strong magnetic changes.Reciprocal space maps (RSMs) obtained in a representative H_L region are shown in Figure S4 for K=0, applying an output laser power of 30 mW.As in Figure 1 in the manuscript, an incommensurate growth of Fe3O4 on BaTiO3 is evident where the integer positions correspond to the reflections of the BaTiO3 crystal and the non-integer positions to the reflections of the Fe3O4 layer.The lattice parameter of Fe3O4 (8.396 Å) 2 is approximately twice the lattice parameter of BaTiO3 (a=3.99916Å and c=4.03630Å). 1 Therefore, the observed reflections of Fe3O4 possess twice the order of the adjacent reflection corresponding to BaTiO3.Note that twin boundaries are observed in all reflections found in this H_L region along the L direction.The figure on the right shows the H_projection of the orange delimited area where the Fe3O4, (8 0 4) and BaTiO3, (4 0 2) reflections are displayed.Table S1 shows the retrieved structural parameters from the RSM data.Table S1: Calculated in-plane and out-of-plane crystalline domain and cell parameters of the Fe3O4 thin film retrieved from RSM analyses.Valuable differences can be observed in the crystalline domain size with light.However, variations in the lattice parameter are negligible, evidencing a dominant magneto-electric coupling.

Light Power
In

Figure S1 :
Figure S1: High resolution X-ray diffraction set-up configurations.a, In-plane and b, out-of-plane -2 scans configurations performed by a six-circle diffractometer in the CRG SpLine Beamline BM25 in Grenoble, France.

Figure
Figure 2f-i of the main manuscript.

Figure S2 :
Figure S2: Magnetic Features of the Multiferroic Fe3O4/BaTiO3 Heterostructure as a Function of the Output Laser Power.Magnetic hysteresis loops are collected by an alternating gradient magnetometer (AGM), applying light with a green laser diode (λ = 532 nm) from 0 to 50 mW along different sample orientations: a, =0°, b, =45°, and c, =90°.A schematic of the ferroelectric domain distribution is displayed on top of each figure.A variation of the remanence and coercivity as a function of the sample rotation angle is observed for each output laser power, showing a uniaxial anisotropy caused by pulsed laser growth (PLD).d, e, and f, Evolution of the HC, MR/MS), and normalized MS, respectively, asa function of the light power.The dashed black line of the panel h delineates the value of Ms in the absence of illumination, which, in our case, equals 1, as the values have been normalized to these dark conditions.At the top of panels f-g, a 2D schematic representation of the measurement conditions is presented, with black, blue, and red dots representing the values obtained for the 0°, 45°, and 90° configurations, respectively.

Figure S3 :
Figure S3: BaTiO3 crystal structures depending on the thermal range.a, Rhombohedral, b, orthorhombic, c, tetragonal, d, cubic crystal structures of BaTiO3 that are found in the thermal range <200 K, 200 K < T < 275 K, 275 K < T < 400 K and T > 390 K, respectively.

Figure S4 :
Figure S4: Stable Incommensurate Growth with Green Light Exposure.Reciprocal space map (RSM) acquired from representative reciprocal H_L region with K=0 (same region as Figure 1d, manuscript).Integer and non-integer numbers correspond to the BaTiO3 and Fe3O4 reflections represented in BaTiO3 reciprocal lattice units (r.l.u.), respectively.Indicated orange region is taken for H_in-plane, manifesting the epitaxy and good crystallinity of the Fe3O4 thin film.